CHAPTER III. 
Geology of the Llano Estacado and the Plateaux between the Mountain Ranges , from 
the Rio Grande to the Colorado. 
Horizontal character of the strata of the Llano.—Section of the strata.—Absence of bluffs.—Character of the strata 
underlying the Llano.—Gypsum and red clays.—Probable Cretaceous age of the upper strata of the Llano.—Fossils.— 
Cretaceous at Dona Ana.—Possible presence of Tertiary strata.—Absence of Jurassic fossils.—Bed sandstone and Gypsum 
formation of Delaware creek and the Pecos.—Gypsum of the Andes and Iowa.—Of Nova Scotia and Virginia.—Of 
Tuscany, Italy.—Extent of the American formation.—Geology of the plains between the Eio Grande and the Guada¬ 
lupe mountains.—Sand-hills.—Saline lakes.—Springs.—Artesian wells. 
The regular horizontal character of the Llano, and the form of its river valleys, are sufficient, 
without further evidence, to satisfy the geologist that it consists of horizontal strata of rock ; a 
conclusion which is supported by the observations of all who have visited the region. The 
sketches of the borders of the Llano, accompanying the report of Captain Marcy, all show, with 
beautiful distinctness, the stratification of the rock in horizontal planes. So also the sketches 
brought in by Lieutenant Whipple exhibit successive layers of rock cropping out on the sides 
of gorges and canons, and again resting upon the tops of mounds, in broad, tahle-like summits. 
Dr. G. G. Shumard, the geologist who accompanied Captain Marcy in his expedition to the 
sources of the Red river, has described the horizontal strata of the Llano, and we are indebted 
to him for the first geological section of the formation. This section was taken at the borders 
of the plateau, near the south fork of the Red river, in latitude 34° 30', where the bluffs have a 
height of 600 feet above the prairie below. The base of the bluffs was formed of a deposite of 
red clay 400 feet thick, with a layer of saccharoid gypsum twenty feet thick above it; this was 
surmounted by strata of grey and yellow sandstone, alternating with thin seams of non-fossil- 
iferous limestone; and the upper stratum, about 100 feet thick beneath the soil, is of drift with 
small boulders. 1 
As on the. line of survey there were no abrupt or precipitous borders to the Llano, the oppor¬ 
tunities for .observing the character of the strata were not as favorable as are presented further 
north. Mr. Byrne, in his Diary, under date of March 6th, mentions the occurrence of gypsum 
along the borders of Delaware creek, and the next day he describes several caves in gypsum. 
Lieutenant Marshall, who examined the Pecos river as high up as the Sacramento, reports that 
the country over which he passed was a rolling prairie, and the soil was a mixture of clay and 
decomposed gypsum, evidently resting on a bed of limestone, and on a conglomerate of lime¬ 
stone and clay, which outcrops at several points along the river. “ On the left or east bank of 
the river is the broad plain of the Llano Estacado, which evidently has for its basis the same 
conglomerate rock mentioned above.” 2 He also found red clay and gypsum on the banks of the 
Sacramento river. Captain Pope also mentions immense outcrops of gypsum and selenite in 
‘ ( bluff banks fifty feet high along the Pecos; ’ ’ and states further, that ‘ ‘ numerous caves of pure 
gypsum of dazzling whiteness within, are found in this gypsum formation, which extends over 
a distance of 150 miles along the route.” 3 From these facts, and the configuration of the Llano, 
I am led to regard its substrata as similar to those further north, where it has been more 
J Eeport of an Exploration of Eed river in 1852, by Captain Marcy. [Appendix D, Geology, p. 168, pi. 10.] 
2 Eeport of Captain Pope, p. 66. -Ibid, p. 28. 
